i86 



BLIND AND EYELESS MOLLUSCA 



Mollusca which hal)ituully creep about half buried in wet sand 

 {Bullia, Sigaretus, Scapliander, Philine), eyes are altogether absent. 

 In some species of Natica and Sigaretus, 

 and in Doris, eyes are developed, but are 

 enclosed in a thick layer of skin, througli 

 which they can proljably do little more than 

 faintly appreciate different degrees of light 

 and darkness. Chiton has cephalic eyes in 

 the eml)ryo, Ijut loses them in the adult 

 stage. The two great Auricula, A. auris 

 Jtulae and A. auris Midar, which habitualh' 

 creep about in the liquid mud of mangrove 

 swamps, ]iave entirely lost their eyes. Cer- 

 tain pelagic Mollusca seem to have a 



Fig. 91. — Sigaretus lae- , .... ., ■, • ^ 



iv>/a<i«s Lam., a species tendency, which is not easily explained, to 

 frequenting wet sand, j^gg their eyes or the power of seeincr witli 



and destitute of ex- mi 7- • ^ n 



ternal eyes ; F, ante- them. lllUS lantllllUC liaS 110 CyCS at all. 



rior portion of foot, pteropoda as a rule have no eyes, and the 



(After bouleyet.) ^ . . 



few that have (Creseis, Cavolinia) possess 

 only certain pigmented spots placed near to the nervous centres. 

 In the Heteropoda, however, and the Cephalopoda, many of which 

 are pelagic, the eyes are unusually large. 



Eyes in Deep-sea and Underground Mollusca. — Deep-sea 

 Mollusca, as a rule, possess no visual organs, or possess them only 

 in a rudimentary state, but this rule has its exceptions. Dr. 

 Pelseneer found ^ no trace of eyes in two species of Pleurotoina 

 from 1850 and 1950 fath., none in a Fossarus from 1400 f., 

 none in a Puncturclla from 1340 f. A remarkable form of Voluta. 

 (Guivillca) from 1600 f. possessed eyes which could hardly be 

 functional, as they were destitute of pigment, and exhibited other 

 changes of structure. On the other hand, it is remarkable to 

 notice that in three different species of Troclius from 450 f., 

 565 f., and 1375 f., the eyes were pigmented and well 

 developed. 



In land Mollusca which live beneath the surface of the ground 

 or in absolute darkness, the eyes are generally more or less 

 modified. Thus in Tcstacella, which usually burrows deeply in 

 the soil, but occasionally emerges into the open air, the eyes are 

 very small, but distinct and pigmented. Our little Caecilianella 

 ^ ' Chfllevger' Reports, Zoology, vol. xxvii. part Ixxiv. p. 3. 



